1. Field of the Invention
The present invention relates to a semiconductor laser device used in optical communications, optical measurements and other types of optical information processing. More particularly, it relates to an edge-emitting periodic gain-type semiconductor laser device easily produced by using an MOCVD technique.
2. Description of the Prior Art
High reliability based on high monochromaticity and coherence of the emitted light is required in the optical communications and optical measurements fields. A distributed-feedback (DFB) semiconductor laser device in which periodic refractive index changes are formed in the current injection portion of the laser cavity oscillates laser light in a single longitudinal mode and the oscillation wavelength remains stable even if the temperature or driving current fluctuates.
FIG. 7 is a cross section taken in the direction of an optical waveguide of a prior art DFB laser device. An n-AlGaAs first cladding layer 602, a non-doped AlGaAs active layer 603, and a p-AlGaAs optical guiding layer 604 are grown in this order on an n-GaAs substrate 601. A diffraction grating 607 having a period nearly matching the wavelength of the laser light (inside the DFB laser device) is formed on the top surface of the p-AlGaAs optical guiding layer 604. A p-AlGaAs second cladding layer 605 and a p-GaAs contact layer 606 are successively grown on the p-AlGaAs optical guiding layer 604 on which the diffraction grating 607 has been formed. In this kind of DFB laser device, light having the wavelength matching the period of the diffraction grating 607 is selectively reflected by the diffraction grating 607 and amplified in the cavity.
FIG. 8A shows the relationship between the wavelength and the reflectance of the light generated in the DFB laser device, FIG. 8B shows the relationship between the wavelength and the waveguide gain of the light generated in the DFB laser device, and FIG. 8C shows the relationship between the wavelength and the device gain of the light generated in the DFB laser device.
In the DFB semiconductor layer device, a periodic diffraction distribution is formed in the current injection portion by the diffraction grating 607 as shown in FIG. 7, and therefore, as shown in FIG. 8A, light with a specific wavelength is selectively reflected. This results in a stable longitudinal mode oscillation. As shown in FIG. 8B, however, a clear wavelength selectivity is not demonstrated with respect to the waveguide gain, and therefore the device gain expressed as the product of the reflectance and the waveguide gain is broad as shown in FIG. 8C.
As a result, the device characteristics are easily perturbed and the oscillation wavelength fluctuates greatly in the DFB semiconductor laser device due to light generated in the cavity with differing phases caused by reflecting it off the end of the device or by return light with differing phases entering from the outside of the cavity. This large fluctuation in the wavelength is not desirable when used in the optical measurements field.
Further, a DFB laser device has equivalent reflectance peaks at two neighboring wavelengths as shown in FIG. 8A, which causes problems with oscillation in essentially two modes. In order to solve these problems of the DFB laser device, a semiconductor laser device having a periodic gain structure has been proposed. Here, we will explain the device proposed by Wei Hsin et al. at the 1987 International Electron Devices Meeting (No. 7, pp. 792-795) as one example.
FIG. 9 shows the structure of the prior art periodic gain-type semiconductor laser device. This kind of periodic gain-type semiconductor laser device is produced as described below. First, a semiconductor multilayer is formed by alternately growing Al.sub.0.3 Ga.sub.0.7 As layers and GaAs layers on a semi-insulating GaAs substrate 801. The period with which the Al.sub.0.3 Ga.sub.0.7 As layers and GaAs layers are alternately formed matches the half-wavelength of the emitted light. Following this, a stripe-shaped mesa multilayer active region 802 is formed by etching this semiconductor multilayer. Next, this multilayer active region 802 is buried by growing an n-Al.sub.0.4 Ga.sub.0.6 As layer 804 on the substrate exposed by etching. A p-Al.sub.0.4 Ga.sub.0.6 As region 805 is formed by diffusing zinc ions in the n-Al.sub.0.4 Ga.sub.0.6 As layer 804 on one side of the multilayer active region 802. As a result, a pn junction is formed in the multilayer active region 802, and a surface-emitting periodic gain-type semiconductor laser device is obtained that emits light in the direction indicated by 806.
FIG. 10A shows the relationship between the wavelength and the reflectance of the light generated in a surface-emitting periodic gain-type laser device, FIG. 10B shows the relationship between the wavelength and the waveguide gain of the light generated in a surface-emitting periodic gain-type laser device, and FIG. 10C shows the relationship between the wavelength and the device gain of the light generated in a surface-emitting periodic gain-type laser device.
In this surface-emitting periodic gain-type semiconductor laser device, a periodic refractive index distribution is formed in the multilayer active region 802 by alternately growing Al.sub.0.3 Ga.sub.0.7 As layers and GaAs layers, whereby light having a specific wavelength is selectively reflected as shown in FIG. 10A.
Recombinations between electrons and holes occur more easily in the GaAs layers than in the Al.sub.0.3 Ga.sub.0.7 As layers in the multilayer active region 802. Therefore, gain is selectively supplied to light having the specific wavelength generated in the active region 802 to form a standing wave in which the maximum amplitude nodes thereof are at the center portion of each of the GaAs layers. As shown in FIG. 10B, this surface-emitting periodic gain-type semiconductor laser device has a steep wavelength selectivity with respect to the waveguide gain. As a result, the device gain, expressed as a product of the reflectance and the waveguide gain, shows a steep peak at a specific wavelength as shown in FIG. 10C.
Like a DFB semiconductor laser device, this surface-emitting periodic gain-type semiconductor laser device has equivalent reflectance peaks at two neighboring wavelengths as shown in FIG. 10A, so that it demonstrates equivalent reflectance peaks at two neighboring wavelengths. However, as shown in FIG. 10B, it demonstrates a peak at one wavelength with respect to the active layer gain, thus resulting in a single peak for the device gain as shown in FIG. 10C.
Since the optical waveguide region is formed by mesa etching in this surface-emitting periodic gain-type semiconductor laser device, it is difficult to form the optical waveguide region longer than 10 .mu.m. When the optical waveguide region is short, the generated light is not stably amplified and emitted light with a stable wavelength cannot be obtained at a large amplification factor.
Further, in this surface-emitting periodic gain-type semiconductor laser device, since zinc ions are diffused from one side of the multilayer active region to form a pn junction in the multilayer active region, it is difficult to accurately position the end of the p-type region in the multilayer active region. Therefore, yield is adversely affected in the production of these devices.